The present invention relates to a micro power converter that includes a coil substrate, a power supply IC chip and other parts mounted on a substrate; and further to a method of manufacturing the micro power converter.
Conventional micro power converters such as DC-DC converters used in micro power supplies have a structure in which a power supply IC chip is mounted on a coil substrate by flip chip bonding or the use of an adhesive, and then is wired with gold wires (bonding wires) followed by sealing with a molding resin. FIGS. 17(a) and 17(b), for example, show a structure of a conventional micro power converter employing adhesive bonding. FIG. 17(a) is a plan view of an essential part and FIG. 17(b) is a sectional view of the essential part taken along the line X-X in FIG. 17(a). In FIG. 17(a), magnetic flux 65, 66 is shown, too.
The coil substrate 200 includes first and second coil conductors 54 and 55, first and second external electrodes 57 and 58, and first and second connection conductors 56 and 59 formed thereon. The coil substrate 200 is Cu-plated for forming first and second connection conductors 56 and 59 on the side faces of first and second through-holes 52 and 53, and forming first and second coil conductors 54 and 55, and first and second external electrodes 57 and 58 on the front and back surfaces of the ferrite substrate 51. The through holes 52 and 53 are bored in advance by a sandblasting technique in the ferrite substrate 51 and are surrounded by the ferrite substrate 51. The first through-holes 52 are formed in the inner region of the ferrite substrate 51. The second through-holes 53 are formed in the peripheral region of the ferrite substrate 51 and surrounded by the ferrite substrate 51. The first and the second external electrodes are formed around the second through-holes 53 and surrounded by the ferrite substrate 51. In the FIGS. 17(a) and 17(b), the reference numeral 60 designates a power supply IC chip, the numeral 61 denotes a pad electrode, the numeral 62 denotes an adhesive, the numeral 63 is a bonding wire, and the numeral 64 is a molding resin.
Japanese Unexamined Patent Application Publication No. 2004-274004 and corresponding U.S. Pat. No. 6,930,584 disclose a micro power converter having a power supply IC chip bonded on a coil substrate by flip chip bonding, in which the length of the coil conductors composing a planar solenoid coil is made longer than a certain value with respect to the width of the magnetic insulator substrate (ferrite substrate) to increase the inductance.
In the structure shown in FIGS. 17(a) and 17(b), the magnetic flux 66 passes through a part of the ferrite substrate 51 located outside the second connection conductor 59 formed at the second through-hole 53, and first and second external electrodes 57, 58, as shown in FIG. 17(a). (Magnetic flux 65 passes through a part located inside of these conductors and electrodes.) As a result, an induced electromotive force (noises) in synchronism with the oscillation frequency is generated between top and bottom of the second connection conductor 59, causing malfunction of the micro power supply.
The above-described malfunctioning can be avoided by the coil substrate 300 shown in FIGS. 18(a), 18(b), and 18(c), in which second through-holes 73 (oblong through-holes) with a slit configuration are provided traversing the side face of the ferrite substrate 71 and passing through the ferrite substrate 71. First and second external electrodes 77, 78, and second connection conductors 79 formed at the second through holes 73 interrupts the magnetic flux that would otherwise pass outside these electrodes and conductors, and the magnetic flux 85 takes a route inside the electrodes and conductors. Therefore, the induced electromotive force generated in the second connection conductors 79 decreases and the noises are reduced. FIG. 18(a) is a plan view of an essential part, FIG. 18(b) is a sectional view of the essential part taken along the line X1-X1 in FIG. 18(a), and FIG. 18(c) is a sectional view of the essential part taken along the line X2-X2 in FIG. 18(a). The reference numeral 80 designates a power supply IC chip, numeral 81, a pad electrode, numeral 82, an adhesive, and numeral 83, a bonding wire. The reference numeral 72 designates a through-hole, 74 and 75 designate first and second coil conductors, while reference numeral 76 designates a first connection conductor.
However, on the surface of the ferrite substrate 71 on which the power supply IC chip 80 is bonded in the structure of FIGS. 18(a), 18(b) and 18(c), the first external electrodes 77 with gold plating are formed up to outermost edge of the ferrite substrate 71. Molding resin 84 is put on the first external electrode 77 in the process of sealing with the molding resin 84. Since the joining interface between the molding resin 84 and the first external electrode 77 has very poor adhesiveness and the interface exposes to outer atmosphere, moisture permeates from the interface between the first external electrode 77 and the molding resin 84 at the outer edge of the ferrite substrate 71. The device also has an external electrode extending up to the edge of the ferrite substrate like FIGS. 18(a) through 18(c). Thus, humidity resistance of the device is degraded.
In view of the above, it would be desirable to solve the above problem and provide a micro power converter in which magnetic flux passing along a route outside of external electrodes and second connection conductors is decreased to reduce noises, and at the same time, adhesiveness of molding resin is preserved to provide excellent humidity resistance. It would further be desirable to provide a method of manufacturing such a micro power converter.